Kinds of Memory

A panel of experts at national Institutes of Health has given this phenomenon a name: age associated memory impairment. Many insights into its causes have come from major advances in the neuro-sciences in the past decade. In terms of neurochemistry, age associated memory repair impairment and the early stages of Alzheimer's disease are virtually indistinguishable. It may turn out that dementias like Alzheimer's are exaggerated forms of natural aging, but way the process runs amok in some people and is relatively benign in others is unknown. Nor do scientists know why age affects secondary memory more than other kinds of memory.

At one time, scientists believed that memory loss was caused by the death of an estimated 100,000 neurons each day. But better techniques for counting neurons have overturned that notion. It now appears that although some neurons do die in areas of the brain important for memory, the loss is more on the order of 100 cells per day. Over the course of five or six decades, these deaths mount up, so cell loss could account for some of the memory decline that comes with aging, explains Larry R. Squire, a neuro-scientist at San Diego Veterans Administration Medical Center. But perhaps a more important factor is that many neurons shrink or atrophy with age. According to one theory, this happens because of a decline in the production of substances called growth factors, which nourish neurons. Although these cells still function, they maintain fewer connections with other neurons.